1. Field of the Invention
Generally this invention pertains to a coherent radar system and more specifically to a means for obtaining high clutter rejection in coherent radars by generating transmitted and local oscillator signals.
2. Description of the Related Prior Art
In the past, most coherent radar transmitted signals were composed of analog generated baseband or low intermediate (IF) signals which were upconverted to the carrier frequency using a local oscillator. High precision could be obtained but there was little flexibility in the radar signals or architecture. With the advent of high-performance/high-speed digital to analog converters, signals can now be generated easily and this allows for new improved radar architectures. However, the signals after digital-to-analog conversion are not pure representations of the desired digital signal. The resulting signal is composed of the desired signal plus low level deterministic spurious signals plus noise. Current direct digital synthesis units have spurious signals on the order of xe2x88x9270 dBc when operating at 20 MHz rates and as low as xe2x88x9240 dBc when operating at 1 GHz rate. When these signals are multiplied up to carrier frequencies these spurious signal are even larger. If the spurious signals are not coherent from pulse-to-pulse, any reflection of them from clutter (i.e., echos with zero Doppler shift) will not cancel in a radar moving target indicator (MTI) system.
The object of the invention is to provide a means of obtaining high clutter rejection in radars using off-the-shelf digital-to-analog converters to generate a transmitted and a local oscillator signals.
This and other objectives of this invention are achieved by a digital coherent radar that generates its transmitted waveform from a low intermediate frequency (IF) and a local oscillator (LO) by digitally generated waveforms after passing through digital-to-analog (D/A) converters. The LO is upconverted using a product multiplier. The IF representation of the transmitted waveform is upconverted using the LO. The transmitted waveform is amplified and passed through a circulator to an antenna. The echos are received through the antenna and passed through the circulator and receiver protector. The received signal is then downconverted to a digital signal and passed on to a processor. Finally, the waveform is reset and restarted by the local oscillator generator, the digital-to-analog converters, and the analog-to-digital converter at the beginning of each pulse. It must be insured that all pulses in a pulse train are identical even though there are deterministic errors in the representation of the transmitted signals desired coherent component. This insures that all transmitted and received signals regardless of their errors in representation (i.e., spurious signal content) have the same exact representation from pulse-to-pulse for clutter like signals. If this is not done, the error type echo signals (i.e., spurious signals) will not be the same every pulse and may not cancel while the intended signal will.